8.2 Surface area and the rate of reaction

• Catalysts work because they lower the activation energy needed to start the reaction
• A catalyst speeds up reactions without being changed itself, so it can be reused
• Surface Area: The total area of a solid or liquid that is exposed to the air or other substances.
• The effect of changing the surface area on the rate of reaction can be observed when comparing the burning of magnesium in different forms. When magnesium ribbon is placed in a Bunsen flame, it reacts rapidly and produces a bright white flame to form magnesium oxide. However, if a large piece of magnesium is used, it does not burn in the same way. Conversely, magnesium powder burns much faster than the ribbon.
• This difference in reaction rate is due to the accessibility of magnesium atoms to oxygen in the air. Only the magnesium atoms on the surface can come into contact with oxygen and react. In a solid block of magnesium, most of the atoms are located inside the block, away from the oxygen. In contrast, the magnesium ribbon has a greater proportion of atoms on the surface, allowing for more efficient contact with oxygen and faster reaction.
• Magnesium ribbon exhibits a larger total surface area compared to a solid block of magnesium with the same mass. Magnesium powder, with its fine particles, has an even larger total surface area. Therefore, it provides the most atoms available to react, resulting in the fastest reaction rate among the three forms of magnesium.
• When: Surface area becomes a critical factor whenever substances engage in chemical interactions, particularly in reactions where only surface atoms or molecules participate.
• What: Surface area pertains to the aggregate area of all exposed surfaces of an object or substance. In reaction kinetics, it dictates the extent to which reactant particles are accessible to one another or to external agents.
• Why: Surface area profoundly influences reaction rates by modulating the accessibility of reactant particles to each other. In reactions where only surface species can partake, augmenting surface area heightens the frequency of collisions among reactants, thereby expediting reaction kinetics.
• How: Altering surface area involves modifying the physical morphology of a substance. Processes such as comminution, pulverization, or thin-film formation increase surface area, while compaction or densification reduces it. Manipulating surface area in reactions generally enhances reaction kinetics by fostering a greater collision frequency among reactant species.
• Surface Area